Original paper Petrology and zircon U–Pb dating combined ... · tinental crust are of great importance in understanding the early evolution of the Earth (Condie 1989, 1994; Rudnick

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Journal of Geosciences, 59 (2014), 275–291 DOI: 10.3190/jgeosci.172

Original paper

Petrology and zircon U–Pb dating combined with Hf isotope study of granitic rocks from the Kuluketage Block (Tarim Craton, NW China)

Qian YUAN 1, 2, Xiao-feng CAO1, 2, Xin-biao LÜ1, 2*, En-lin YANG1, Xiang-dong WANG1, Yue-gao LIU1, Ban-xiao RUAN1, Munir MOHAMMED-ABDALLA-ADAM1

1 Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China2 State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China; [email protected]* Corresponding author

We report the petrology, whole-rock geochemistry, zircon LA-ICP-MS U–Pb chronology and zircon Hf isotopic data of Daxigou granitoids (western part of the Kuluketage Block, NW China) to evaluate their likely petrogenesis and tectonic setting. Zircons from syenogranite can be divided into two groups: 1) those that display oscillatory zoning and high Th/U ratios (average = 1.38), implying their magmatic origin and 2) those that exhibit weak zoning and extremely high U and Pb contents but low Th/U ratios (average = 0.35), resembling zircons that experienced hydrothermal alteration. The zircon LA-ICP-MS U–Pb dating of the two groups of zircons yielded weighted mean ages of 1830 ± 12 Ma (MSWD = 0.78) and 1798 ± 21 Ma (MSWD = 1.6) respectively. The Daxigou granitoids belong mostly to normal-K and sodium-rich metaluminous calc-alkaline type, systematically enriched in LREE and large ion lithophile elements (LILE, e.g., K, Ba and Rb), but significantly depleted in high field strength elements (HFSE, e.g., Ti, P, Nb, Ta and U). Their εHf(t) values and two-stage Hf model ages range from –7.16 to –5.03 and 2.69 to 2.76 Ga, respectively. Taken together, it is suggested that Daxigou granitoids are of I-type affinity and that they were derived by partial melting of a Neoarchaean TTG (e.g., Tuoge Complex) rocks in a continental-arc environment. These new data, combined with previous regional geological studies, demonstrate that a series of Palaeo-proterozoic (c. 2.0–1.8 Ga) tectono-magmatic events occurred in Kuluketage Block during the assembly of Columbia.

Keywords: Kuluketage Block, syenogranite, LA-ICP-MS zircon dating, Hf isotopes, PaleoproterozoicReceived: 7 June 2013; accepted: 16 June 2014; handling editor: M. Kohút

1. Introduction

The formation and reworking of early Precambrian con-tinental crust are of great importance in understanding the early evolution of the Earth (Condie 1989, 1994; Rudnick 1995; Hawkesworth and Kemp 2006; Long et al. 2010). The Tarim, as well as the North and South China cratons, constitute three major continental blocks in China and represent an important part of the early crustal evolutionary history of northwest China and adjacent areas (Hu AQ et al. 1997; Lu et al. 2008; Demoux et al. 2009; Xiao and Kusky 2009; Lei et al. 2012). The Tarim Craton has a poorly dated Archaean–Paleoproterozoic basement which sporadically crops out along the margins of the Mesozoic–Cenozoic Tarim Basin (Lu et al. 2002). By contrast, the Kuluketage Block (also spelled as Ku-ruqtagh or Quruqtagh) on the northeastern margin of the Tarim Craton (Fig. 1a–b) is predominantly composed of the Precambrian basement (Lu et al. 2002, 2008; Wang et al. 2013) and provides a good opportunity to study the Precambrian evolutionary history of the Tarim Craton.

Several tectono-thermal events from Neoarchaean to the latest Neoproterozoic have been determined in this area. However, most of the previous studies have mainly focused on Neoproterozoic magmatism and tec-tonic evolution related to the break-up of Rodinia (Xu et al. 2005; Luo et al. 2007; Sun and Huang 2007; Lu et al. 2008; Zhu et al. 2008; Zhang et al. 2009; Shu et al. 2010; Cao et al. 2011), e.g., c. 800–820 Ma Qiganbulake mafic–ultramafic–carbonatite complex, 820 ± 10 Ma Xingdi granodiorite, 795 ± 10 Ma Taiyangdao granite (Zhang et al. 2007a), c. 755 Ma bimodal volcanic rocks in the Xinger area (Xu et al. 2005) and 630–650 Ma mafic dykes in Korla (Zhu et al. 2008).

By contrast, little is known about pre-Neoproterozoic magmatism (especially for Mesoproterozoic–Paleopro-terozoic magmatism) and the tectonic evolution of the Kuluketage Block. A few studies that have reported on the Paleoproterozoic magmatism and tectonic evolution in the area, mainly dealt with Nd model ages (Feng et al. 1995) and ages from zircons, detrital (Guo et al. 2003; Hu and Wei 2006; Long et al. 2010; Shu et al.

Qian Yuan, Xiao-Feng Cao, Xin-Biao Lü, En-Lin Yang, Xiang-Dong Wang, Yue-Gao Liu, Ban-Xiao Ruan, Munir Mohammed-Abdalla-Adam

276

2010) or metamorphic (Lei et al. 2012). However, no Paleoproterozoic ages of magmatic zircons have been reported yet. Moreover, detailed field observations, precise isotopic ages and high-quality geochemical data are very sparse for late Paleoproterozoic rocks, and this hinders a good understanding of the tectonic evolution of the Tarim Craton, especially the tectonic setting of the Paleoproterozoic magmatism and its relationship to the Palaeo–Mesoproterozoic Columbia Supercontinent (Lei et al. 2012).

Based on detailed field and petrological studies, we report comprehensive geochronological, geochemical and zircon Hf isotope analyses of the Daxigou Com-plex (syenogranite and granodiorite) in the Kuluketage Block with the aim of characterising its petrogenesis and tectonic setting. Together with regional geology and geochronological data, the Palaeo–Mesoproterozoic as-sembly of Columbia is being investigated.

2. Geological setting

The Kuluketage Block is composed of two units: the basement which includes Archaean, Paleoproterozoic, Mesoproterozoic and early Neoproterozoic lithologies and the middle Neoproterozoic to Phanerozoic sedi-mentary cover (Gao et al. 1993; Cheng 1994; Feng et al. 1995; Lu et al. 2008) (Fig. 1b). The Xinger and Xingdi faults are the main regional E–W-oriented structures (Fig. 1c).

Archaean rocks sporadically crop out in the Kuluket-age area, known as the Tuoge Complex. It is mainly composed of granitic gneisses with minor amphibolite xenoliths (derived from gabbroic protoliths) (Hu AQ et al. 1999, 2000); it yielded a SHRIMP U–Pb zircon age of 2601 ± 21 Ma (Zhang et al. 2012a) and LA–ICP–MS U–Pb upper intercept zircon age of 2659 ± 15 Ma (Long et al. 2011a). Mostly metasedimentary

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00 ′ 00 ′ 00 ′ 00 ′

00 ′00 ′00 ′00 ′ 30 ′30 ′ 30 ′ 30 ′

30 ′ 30 ′ 30 ′ 30 ′

30 ′

30 ′

0 20 40 km

0 200 400 km

N

Xinger Fault

TaiyangdaoSaimashanFig 2.

Qieganbulake

No 3.

Xingdi Fault

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Mountain

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Qie

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Kashi

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Phanerozoic rocks Neoproterozoic rocks Mesoproterozoic rocks

Palaeoproterozoic rocks Archaean rocks Mafic–ultramafic rocks

Early Neoproterozoic granitoids

Unclassified granitoid rocks

Faults Inferred faults

Nanhua and Sinian tillite

Quaternary desert andsedimentary deposits

AltynTagh

Mountain

KuokexiKuokexi

Fig. 1a – Main tectonic domains of China (simplified from Cao et al. 2011). NCB: North China Block, SCB: South China Block, SGT: Song-pan–Ganzi Terrane, QB: Qaidam Basin, QT: Qiangtang Terrane, LT: Lhasa Terrane; b – Schematic geological map of Tarim Craton and adjacent areas (adopted from Cao et al. 2011); c – Schematic geological map of the Kuluketage Block on the northeast margin of the Tarim Craton (Wang et al. 2007).

Petrogenesis and tectonic setting of granitoids from the Daxigou Complex, Kuluketage Block, NW China

277

Paleoproterozoic rocks (Xingditage Complex) occur in the western part of the Kuluketage Block. Despite the limited precise geochronology, at the end of the Paleoproterozoic, an important metamorphic event was postulated to have affected Archaean TTG suites and the overlying Paleoproterozoic sedimentary rocks (e.g., Feng et al. 1995; Lu et al. 2002; Zhang et al. 2007b). Mesoproterozoic to early Neoproterozoic low-grade metamorphic rocks, including metamorphosed carbonate and clastic metasedimentary rocks, as well as granitoids, are widespread in the area (Feng et al. 1995; Lu et al. 2008). Middle Neoproterozoic to Phanerozoic rocks consist of mafic dyke swarms, bimodal volcanics as well as fine sandstones, siltstones, shales, dark limestones and chert nodule-bearing dolomites. The mafic dykes with bimodal volcanics were formed at 820–744 Ma and 650–630 Ma (Zhang et al. 2007a; Zhu et al. 2008; Xu et al. 2009). Late Neoproterozoic glacial deposits are also well exposed (Xu et al. 2009).

The Daxigou Complex is the first low-grade, large iron–phosphate deposit discovered at the northern margin of the Tarim Craton (Xia et al. 2010). Several similar complexes with Fe–P mineralization are located along the Xingdi Fault (Xia et al. 2010) and form a strong linear aeromagnetic anomaly (Yuan et al. 2013). From west to east, the petrology of these igneous bodies is as follows: Duosike pyroxenite Complex, Kawuliuketage pyrox-enite–hornblendite–syenite Complex, Ao’ertang gabbro–pyroxenite Complex, Daxigou granodiorite–syenogranite Complex and Qieganbulake biotite pyroxenite–carbonate Complex (Xia et al. 2010).

3. Field geology and petrography

The Daxigou Complex is located south of the Xingdi Fault in Kuluketage Block (Fig. 1c) and is controlled by its subsidiary fault. The coordinates of the work-ing area are 87°27'00"–87°31'00"E and 41°12'30"–41°15'30"N. Rocks of this complex are distributed NW–SE over an area of 2.1 × 0.9 km (Fig. 2). They intruded into the Archaean Tuoge Complex which is mainly composed of amphibole-bearing tonalitic gneiss (Xia et al. 2010).

The Daxigou Complex is built mainly by greyish white granodiorite (GD) that exhibits coarse-grained granitic texture and blocky structure. The main mineral compo-nents are plagioclase (45–50 vol. %), quartz (25–30 vol. %), K-feldspar (14–17 vol. %), hornblende (8 vol. %), magnetite (1–2 vol. %) and apatite (1 vol. %). Accessory minerals include zircon and ilmenite.

Syenogranites (SG) occur mainly as dykes accom-panying the granodiorite, and account for 30 % of the complex. Typically they are pinkish and show massive, medium- to coarse-grained granitic textures. Modal compositions include plagioclase (An35–50 30–50 vol. %), quartz (25–35 vol. %), K-feldspar (18–22 vol. %), biotite (2–5 vol. %), hornblende (1–2 vol. %), and accessory minerals such as apatite, zircon and ilmenite.

Rocks of the Tuoge Complex are composed of pla-gioclase (40–48 vol. %), quartz (25–30 vol. %), alkali feldspar (25–30 vol. %) and biotite (5–10 vol. %) with some accessory minerals, e.g., titanite, apatite and zircon. Generally, the Tuoge Complex exhibits considerable

72 ArZK2 3-

ZK2 1-

ZK2 2-

ZK0 2-

ZK0 3-

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ZK1 5-

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Fig. 2 Geological map of the Daxigou iron–phosphorite deposit in the western Kuluketage (Xia et al. 2010) showing the locations of geochronological and geoche-mical samples (SG-1 to SG-4: syenogranite; GD-1 to GD-3: granodiorite).


278

weathering in the Daxigou area, rendering it not suitable for chemical analyses.

4. Analytical methods

4.1. Zircon U–Pb dating

Zircon grains were separated using conventional crush-ing, grinding and wet shaking table methods, followed by heavy liquid (tetrabromomethane) and magnetic separation. Hand-picked zircon grains were mounted in epoxy blocks and polished prior to LA-ICP-MS analysis, the surfaces of grain mounts were washed in dilute HNO3 and pure alcohol to remove any potential lead contamination. The selection of zircon grains for isotopic analyses was based upon cathodoluminescence (CL) images (Fig. 3). Zircon U–Th–Pb measurements were done at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan (GPMR-CUGW), using a GeoLas 2005 System. An Agilent 7700a ICP-MS instrument was employed, with a 193 nm ArF-excimer laser (32 μm beam diameter). Details on instrumentation and ana-lytical accuracy were given by Liu et al. (2008, 2010). Time-dependent drifts of U–Th–Pb isotopic ratios were corrected using a linear interpolation (with time) for ev-ery five analyses according to the variations of external zircon standard 91500 (i.e., 2× 91500 – 5 samples – 2× 91500) (Liu et al. 2010). The ages were calculated by in-house software ICPMSDataCal (ver 9.0) (Liu et al. 2008), and Concordia diagrams were plotted by Isoplot/Ex ver. 3.0 (Ludwig 2003).

4.2. Whole-rock geochemistry

Rock samples for the major- and trace-element analy-ses were carefully selected to be representative of geographical distribution of the two different rock types (Fig. 2): four syenogranites and three grano-diorites. Whole-rock samples were crushed to 0.5 cm chips in a steel-faced jaw crusher and powdered with an agate mill.

Major elements were analysed with a PAN analytical Axios X-ray fluorescence spectrometer (XRF) at ALS Chemex (Guangzhou) Ltd. A calcined or ignited sample (0.9 g) was added to 9.0 g of lithium borate flux (1 : 1 Li2B4O7–LiBO2), mixed well and fused in an auto fluxer between 1050–1100 °C. A flat molten glass disc was prepared and analysed by XRF with a precision better than 5 %.

Trace-element concentrations were determined with an Elan 9000 ICP-MS at the same lab. To the sample powder (0.2 g) was added lithium metaborate flux (0.9 g), mixed well and fused in a furnace at 1000 °C. The resulting melt was then cooled and dissolved in 100 ml of 4 % HNO3/2 % HCl solution and analyzed by ICP-MS with a precision better than 10 % for all elements.

4.3. In situ zircon Hf isotope analysis

In situ zircon Hf isotopic analyses were conducted us-ing a Neptune Plus MC-ICP-MS, in combination with a Geolas 2005 excimer ArF laser-ablation system, at the GPMR-CUGW. During the analysis, a laser repetition rate of 20 Hz at 200 mJ was used with the spot diam-eter of 44 μm. Details of the analytical technique were described in (Hu ZC et al. 2012). During the analysis, the 176Hf/177Hf ratios of the standard zircon (GJ-1)

were 0.282013 ± 0.000022 (2σ, n = 276), agreeing with the rec-ommended values (Woodhead and Hergt 2005; Wu FY et al. 2006; Sláma et al. 2008; Li et al. 2010) within 2σ error. Off-line selection and integration of analytical signals, and isobaric interference and mass fraction-ation correction of Lu–Hf isoto-pic ratios were also performed by the ICPMS-DataCal.

GO 3-GO 1- GO 4-

GO 5-

GO 6-

GO 7- GO 8- GO 9- GO 10- GO 11-

GT 1- GT 2- GT 3-GT 4- GT 5- GT 6- GT 7- GT 8-

100 mμ

GO 2-

-5.14

- .6 69- .6 14

- .6 58

- .5 82

- .6 04

- .6 57

- .6 61- .5 97

- .5 03 - .6 18

- .6 92 - .6 77- .5 96

- .6 93- .7 10 - .6 51

- .6 58 - .7 16

1849 ± 18 Ma 1834 ± 18 Ma

1774 ± 17 Ma1792 ± 19 Ma

1829 ± 26 Ma 1821 ± 24 Ma

1799 ± 20 Ma

1834 ± 21 Ma

1869 ± 22 Ma

1813 ± 22 Ma

1770 ± 21 Ma1776 ± 22 Ma

1830 ± 22 Ma

1824 ± 20 Ma

1820 ± 22 Ma1835 ± 16 Ma

1796 ± 19 Ma 1828 ± 17 Ma

1827 ± 19 Ma

Fig. 3 Cathodoluminescence (CL) im-ages of zircons from the syenogranite SG-2. LA-ICP-MS U–Pb (red circles) and in situ Hf determination spots (big-ger circles) with 207Pb/235U ages and εHf(t) values are indicated.


279

5. Analytical results

5.1. Zircon U–Pb geochronology

Zircons are relatively abun-dant in the dated syenogranite sample SG-2 (Fig. 2). Accord-ing to the shape, colour and length/width ratios, they can be categorised into two groups, distinguished below. The mea-sured Pb isotopic ratios and calculated ages for 19 analyses on 19 zircon crystals are given in Tab. 1.

5.1.1. Group one (igneous) zircons

Group one (GO) zircons are sub-euhedral, short to long pris-matic, and transparent, and their length/width ratios are c. 2.7. In CL images (Fig. 3; upper two rows), they exhibit oscil-latory zoning, a feature typical of magmatic zircon (Corfu et al. 2003), but a few grains show clear core–rim textures (GO-6).

The analyses display vari-able abundances of U (99–513 ppm), Th (123–622 ppm) and Pb (49–256 ppm) but consis-tently high Th/U ratios (1.09–2.11) which are suggestive of an igneous origin (Hanchar and Rudnick 1995; Hoskin and Black 2000). This group of closely clustered concordant analyses yields a weighted mean 207Pb/235U age of 1830 ± 12 Ma (MSWD = 0.78) (Fig. 4a), which we adopt as the crystallization age of the syenogranite.

5.1.2. Group two (recrystallized) zircons

By contrast, group two (GT) zircons are irregular in shape

Tab.

1 L

aser

-abl

atio

n IC

P-M

S U

–Pb

isot

opic

dat

a fo

r zirc

on fr

om th

e da

ted

syen

ogra

nite

Sam

ple

spot

Con

cent

ratio

ns (p

pm)

U–T

h–Pb

isot

opic

ratio

sA

ges

(Ma)

PbTh

UTh

/U20

7 Pb/

206 P

b1σ

207 P

b/23

5 U1σ

206 P

b/23

8 U1σ

207 P

b/20

6 Pb

1σ20

7 Pb/

235 U

1σ20

6 Pb/

238 U

1σG

O-1

152.

055

326

22.

110.

1088

0.00

225.

1793

0.10

670.

3411

0.00

3017

8936

1849

1818

9214

GO

-214

5.4

363

282

1.29

0.10

560.

0022

5.08

900.

1078

0.34

520.

0031

1726

3918

3418

1912

15

GO

-349

.713

099

1.31

0.10

680.

0034

5.06

000.

1560

0.34

040.

0039

1746

5718

2926

1889

19

GO

-425

6.0

622

513

1.21

0.10

360.

0029

5.00

810.

1391

0.34

540.

0034

1700

5218

2124

1912

16

GO

-516

9.1

457

325

1.41

0.10

570.

0027

5.08

510.

1275

0.34

420.

0035

1728

4618

3421

1907

17

GO

-666

.918

112

91.

400.

1063

0.00

275.

0659

0.13

180.

3407

0.00

3617

3747

1830

2218

9017

GO

-715

1.8

455

286

1.59

0.10

710.

0025

4.88

260.

1138

0.32

550.

0028

1752

4417

9920

1816

14

GO

-810

5.7

246

218

1.13

0.11

070.

0029

4.96

280.

1263

0.32

070.

0031

1811

4818

1322

1793

15

GO

-913

1.8

340

260

1.31

0.10

920.

0026

5.02

500.

1183

0.32

870.

0029

1787

4318

2420

1832

14

GO

-10

55.6

123

113

1.09

0.11

290.

0030

5.30

330.

1385

0.33

730.

0036

1847

4818

6922

1873

17

GO

-11

128.

533

325

71.

300.

1113

0.00

255.

0454

0.11

380.

3257

0.00

3018

2036

1827

1918

1815

Ave

rage

128.

4 3

45.7

249.

51.

38

GT-

182

0.0

293

2248

0.13

0.10

410.

0021

4.73

910.

0965

0.32

570.

0029

1698

6917

7417

1817

14

GT-

248

8.0

598

1216

0.49

0.10

560.

0023

4.84

280.

1084

0.32

800.

0031

1726

4117

9219

1828

15

GT-

335

6.3

392

841

0.47

0.10

380.

0028

5.00

670.

1323

0.34

440.

0034

1692

4918

2022

1908

16

GT-

495

2.0

546

2411

0.23

0.10

560.

0024

4.86

380.

1094

0.32

880.

0030

1724

4117

9619

1833

14

GT-

511

46.1

493

3024

0.16

0.10

520.

0027

4.74

830.

1224

0.32

170.

0034

1718

4717

7622

1798

17

GT-

624

2.2

302

569

0.53

0.10

910.

0024

5.04

940.

1114

0.33

080.

0030

1785

4018

2819

1842

15

GT-

720

3.8

290

466

0.62

0.10

870.

0027

5.09

340.

1284

0.33

490.

0034

1789

5118

3521

1862

16

GT-

893

6.0

471

2354

0.20

0.10

890.

0028

4.71

510.

1199

0.31

000.

0033

1781

1417

7021

1741

16

Ave

rage

643.

1 4

23.1

1641

.10.

35

GO

= g

roup

one

; GT

= gr

oup

two


280

and orange in colour under the optical microscope; their length/width ratios are c. 1.2. In CL images (Fig. 3, the bottom row), they are much darker than magmatic rims and exhibit no zoning. This suggests that they may have

undergone hydrothermal alteration, similar to the zircons in alkali syenites (Corfu et al. 2003).

The group two zircons show much higher U (466–3024 ppm), Pb (204–1146 ppm) and total REE contents,

but lower Th/U ratios (0.13–0.62) than those of group one (Tab. 1). However, they show 176Lu/177Hf and 176Hf/177Hf(t) ratios identical to the GO zir-cons (see Tab. 3). These char-acteristics are similar to those of zircons formed by alteration with an aqueous fluid or a hy-drous melt (e.g., Gerdes and Zeh 2009). Eight analyses of eight irregular grains with bad oscillatory zoning yielded a weighted mean 207Pb/235U age of 1798 ± 21 Ma (MSWD = 1.6, 2σ) (Fig. 4b), i.e. postdating by nearly 30 Ma the intrusion (GO). Thus we interpreted this datum as the age of post-mag-matic alteration.

5.2. Major elements

The representative whole-rock major- and trace-element com-positions are given in Tab. 2, including those for the Tuoge Complex. In addition, these samples may have undergone some degree of alteration, such as chloritization, even though their LOI values are moderate (2.09–3.63 wt. %), except the sample SG-3 (6.58 wt. %).

The syenogranites are char-acterised by variable SiO2 (60.44–73.28 wt. %), K2O (1.05–6.23 wt. %), high Na2O (3.17–5.62 wt. %), and low P2O5 (0.014–0.113 wt. %), TiO2 (0.03–0.43 wt. %) with MgO (0.24–1.66 wt. %). After re-jection of the K-rich sample (SG-1), the Na2O/K2O ratios range from 1.19 to 5.35, i.e. are characteristic of I-type granites

Data-point error ellipses are 2σ

20

6P

b/

23

8U

207 Pb/ 235U

(a)

(b)

3 5. 4 5. 5 5. 6 5.0 28.

0 30.

0 32.

0 34.

0 36.

0 38.

1940

1900

1860

1820

1780

1740

Box heights are 2σ

Mean 1830 95 conf

Wtd by data pt errs only 0 of 11 rej

MSWD 0 78 probability 0 65

error bars are 2

= % .

- , .

= . , = .

( )σ

Data-point error ellipses are 2σ

Box heights are 2σ

2100

2000

1700

1600

2100

2000

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19001900

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0 30.

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1860

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20

6P

b/

23

8U

207 Pb/235

U

Mean 1798 95 conf

Wtd by data pt errs only 0 of 8 rej

MSWD 1.6 probability 0 12

error bars are 2

= % .

- , .

= , = .

( )σ

18001800

19001900

18001800

Fig. 4 U–Pb Concordia plots and recal-culated weighted mean 207Pb/235U ages of group one zircons (a) and group two zircons (b) are given in the text box.


281

Tab. 2 Major- (wt. %) and trace-element data (ppm, including REE) from the Daxigou granitoids (this work) and Tuoge Complex (Long et al. (2010)

Sample No. SG-1 SG-2 SG-3 SG-4 GD-1 GD-2 GD-3 TC-1 TC-2 TC-3 TC-4Lithology Syenogranite Granodiorite Tuoge ComplexSiO2 69.83 73.28 60.44 68.86 69.25 72.05 61.85 69.30 68.90 71.00 66.30Al2O3 14.22 12.23 14.46 12.75 12.61 12.43 14.98 13.50 13.60 13.10 14.40Fe2O3(T) 0.52 3.38 4.38 1.60 5.08 2.70 6.90 4.06 4.50 3.69 4.94CaO 2.17 0.96 4.58 4.23 1.46 2.07 3.59 2.57 2.50 0.98 2.74MgO 0.24 1.40 1.66 0.39 1.79 0.91 2.30 0.97 1.36 1.26 1.71Na2O 3.17 3.87 5.62 4.05 4.45 5.16 4.21 3.72 3.63 4.54 3.30K2O 6.23 2.59 1.05 3.40 1.49 1.43 1.02 3.00 2.68 3.34 3.43TiO2 0.03 0.08 0.43 0.03 0.22 0.26 0.94 0.81 0.86 0.56 0.89MnO <0.01 0.05 0.07 0.02 0.07 0.04 0.06 0.08 0.08 0.09 0.07P2O5 0.01 0.03 0.11 0.02 0.09 0.09 0.35 0.25 0.25 0.19 0.31SrO 0.02 0.02 0.04 0.03 0.02 0.03 0.07 – – – –BaO 0.13 0.07 0.08 0.09 0.10 0.05 0.10 – – – –LOI 2.46 2.09 6.58 3.63 2.57 2.44 2.80 1.23 1.10 0.83 1.32∑ 99.02 100.05 99.50 99.10 99.19 99.65 99.17 99.50 99.50 99.60 99.40Na2O/K2O 0.51 1.49 5.35 1.19 2.99 3.61 4.13 1.24 1.35 1.36 0.96Na2O+K2O 9.40 6.46 6.67 7.45 5.94 6.59 5.23 6.72 6.31 7.88 6.73A/NK 1.19 1.33 1.39 1.23 1.41 1.24 1.87 1.44 1.53 1.18 1.58A/CNK 0.89 1.12 0.77 0.71 1.09 0.90 1.03 0.96 1.01 1.02 1.02δ 3.29 1.38 2.55 2.15 1.34 1.49 1.45 1.72 1.54 2.22 1.94Co 82.50 78.20 33.70 64.40 22.60 46.70 41.90 4.38 6.58 6.34 6.67Ni 10.00 9.00 17.00 8.00 10.00 10.00 26.00 5.67 3.82 6.65 7.39Rb 126.5 52.9 23.2 67.8 30.5 30.2 13.4 48.6 61.7 39.9 69.5Ba 1205 699 739 828 957 554 887 2206 2044 1974 2586Th 0.58 4.14 4.77 3.42 8.4 4.62 0.47 7.81 7.80 11.07 14.96U 0.25 0.59 0.69 1.01 0.51 0.47 0.44 0.52 0.52 0.44 0.50K 51715.9 21499.9 8716.2 28223.7 12368.6 11870.6 8467.1 24911.3 22254.1 27734.6 28481.9La 3.6 9.2 38.8 6.8 28.0 21.8 35.3 108.0 116.0 110.0 153.0Ce 6.6 15.5 70.7 13.1 53.8 41.7 78.1 236.0 250.0 232.0 304.0Pb 18.00 10.00 6.00 13.00 5.00 9.00 25.00 17.58 10.97 9.93 13.23Pr 0.65 1.62 7.33 1.52 5.80 4.36 9.77 28.00 29.70 28.00 32.30Sr 223 206 370 283 207 241 682 346 265 140 436P 61.50 140.57 496.40 96.64 421.72 395.36 1550.69 1091.34 1091.34 829.42 1353.26Nd 2.30 6.00 26.60 5.90 20.70 16.20 41.00 102.00 106.00 98.70 109.00Ta 0.40 0.30 0.50 0.50 0.30 0.30 0.30 1.14 0.77 0.53 0.65Zr 41 45 209 46 145 143 281 408 253 300 310Hf 1.70 1.30 5.40 1.70 4.20 4.60 6.60 10.25 6.23 7.57 7.70Sm 0.45 1.04 4.26 1.42 3.81 3.27 7.34 17.20 16.40 15.00 13.20Eu 0.50 0.51 1.16 0.75 1.09 0.81 2.06 3.53 3.37 2.63 2.70Ti 143.65 383.06 2058.96 143.65 1053.42 1244.95 4500.98 4856.25 5156.01 3357.40 5335.88Gd 0.36 1.05 3.64 1.60 3.15 4.01 6.09 14.10 13.00 10.50 10.00Tb 0.05 0.18 0.46 0.26 0.48 0.64 0.72 2.14 1.79 1.49 1.05Dy 0.25 1.03 2.54 1.56 3.06 3.66 3.56 12.00 9.65 7.92 4.98Y 1.6 5.9 14.2 10.6 16.8 19.5 16.2 59.5 46.9 38.1 22.7Nb 1.0 1.4 9.8 4.3 4.1 5.9 5.4 18.5 18.1 11.6 10.1Ho 0.05 0.21 0.47 0.33 0.60 0.69 0.60 2.38 1.81 1.47 0.89Er 0.14 0.64 1.34 1.07 1.65 2.03 1.62 6.42 4.63 3.68 2.21Tm 0.03 0.09 0.20 0.16 0.28 0.29 0.20 0.92 0.62 0.47 0.30Yb 0.15 0.69 1.32 1.13 1.60 1.93 1.16 5.88 3.80 2.71 1.77Lu 0.03 0.10 0.20 0.19 0.24 0.28 0.16 0.86 0.53 0.35 0.26ΣREE 15.16 37.86 159.02 35.79 124.26 101.67 187.68 539.43 557.30 514.92 635.66LREE 14.10 33.87 148.85 29.49 113.20 88.14 173.57 494.73 521.47 486.33 614.20HREE 1.06 3.99 10.17 6.30 11.06 13.53 14.11 44.70 35.83 28.59 21.46LREE/HREE 13.30 8.49 14.64 4.68 10.24 6.51 12.30 11.07 14.55 17.01 28.62LaN/YbN 17.22 9.56 21.08 4.32 12.55 8.10 21.83 13.17 21.90 29.12 62.00δEu 3.80 1.49 0.90 1.52 0.96 0.68 0.94 0.69 0.71 0.64 0.72Note: data of Tuoge Complex are from Long et al (2010).A/NK = molar ratio of Al2O3/(Na2O + K2O); A/CNK = molar ratio of Al2O3/(CaO + Na2O + K2O) (Shand 1943); δ = [w(K2O + Na2O)2]/[w(SiO2 – 43)] (Rittmann 1953); δEu = Eu/Eu* = EuN/√SmN + GdN


282

(Chappell and White 1974). Silica alkalic indexes (δ) [wt. % (K2O + Na2O)2] / [wt. % (SiO2 – 43)] (Rittmann 1953) range from 1.37 to 3.29, which suggests calc-alkaline characteristics. In addition, most of the data fall in the medium-K, calc-alkaline field on the SiO2 versus K2O diagram of Peccerillo and Taylor (1976) (Fig. 5a). The rocks are metaluminous, with the A/CNK [molar Al2O3 /(CaO + K2O + Na2O)] varying from 0.71 to 0.93, except sample SG-2 (A/CNK = 1.12) (Fig. 5b, Shand 1943).

The Daxigou granodiorites contain 61.85–72.05 wt. % SiO2, 12.61–14.98 wt. % Al2O3, 1.46 %–3.59 wt. % CaO, 0.91–2.3 wt. % MgO and 1.02–1.49 wt. % K2O, with Na2O/K2O ratios of 2.99–4.13 and low silica alka-lic indexes (δ = 1.34–1.49). All granodiorites belong to the calc-alkaline series (Fig. 5a) and are subaluminous (A/CNK = 0.90–1.09) (Fig. 5b).

The Tab. 2 and Harker plots (Fig. 6) show that the av-erage compositions of syenogranite and granodiorite are similar, with small differences in Fe2O3(t) content. More-over, compared with the Tuoge Complex, the Daxigou granitoids contain rocks with a slightly higher Na2O and a lower FeOt contents; SiO2, Al2O3, CaO and MgO contents are comparable. In the Harker diagrams, the three rock types show consistent negative correlations between SiO2 and Al2O3, Fe2O3t, CaO, MgO and P2O5. Laboratory stud-ies have shown the different behaviour of apatite in I-type (Wolf and London 1994) and S-type granites, and this has been successfully used to distinguish granite types (Chappell 1999). Most of our data show that Daxigou granitoids are metaluminous, and the content of P2O5 is low and negatively correlated with SiO2 (Fig. 6), which corresponds to the evolutionary trend of I-type granites (Chappell and White 1992). Therefore, we suggest that Daxigou granitoids are of I-type affinity and may have a genetic relationship with the Tuoge Complex.

5.3. Trace elements

The trace-element concentrations of the Daxigou granit-oids are highly variable. However, most show mutually comparable patterns in primitive mantle-normalized spi-der diagram (Fig. 7a). Most of the trace-element contents of granodiorites are higher than those of syenogranites but lower than those of the Tuoge Complex. Generally, all the samples are enriched in large ion lithophile ele-ments (LILE, e.g., K, Ba and Rb) but depleted in high field strength elements (HFSE, e.g., Ti, P, Nb, Ta and U) (Fig. 7a), and thus show distribution patterns resembling volcanic-arc rocks. We suggest that Ba was elevated by either K-feldspar or biotite accumulation or, along with Rb and K, during hydrothermal alteration.

The chondrite-normalised REE patterns (Fig. 7b) for the granodiorites and the Tuoge Complex have weak to moderate negative Eu anomalies (Eu/Eu* = 0.64–0.96, calculation method in Tab. 2), whereas the syenogranites show weak negative to moderately positive Eu anomalies (Eu/Eu* = 0.90–3.80). Nevertheless, most samples share similar chondrite-normalised REE patterns enriched in LREE over HREE (Fig. 7b).

5.4. In situ zircon Hf isotopic compositions

The zircons of both groups were analysed for their Lu–Hf isotopic compositions on the dated domains (Fig. 3), and the data are presented in Tab. 3 and graphically illustrated in Fig. 8. Table 3 shows that the 176Lu/177Hf ratios of all zircons are less than 0.002, which indicates that they ac-cumulated little radiogenic Hf since they formed.

Eleven analyses obtained from the GO zircons yielded rather variable εHf(t) values of –6.69 to –5.03 (Tab. 3),

0.5 1.0 1.5 2.00.5

1.0

1.5

2.0

2.5

3.0

A CNK/

AN

K/

Peralumious

Peralkaline

Metaluminous

(b)

40 45 50 55 60 65 70 75 800

1

2

3

4

5

6

7Syenogranites

Granodiorites

Tuoge Complex

(a)

Shoshonite series

High K

calc alkaline

series

-

-

Calc alkaline series

-

Low K tholeiite-

SiO2

KO

2

Fig. 5a – Diagram of K2O–SiO2 for granitoids of the Daxigou and Tuoge complexes (Peccerillo et al. 1976). b – Diagram of A/NK– A/CNK for the same rocks (Shand 1943).


283

55 60 65 70 75 80 85 90

Al O 2

3

55 60 65 70 75 80 85 900

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4

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8

10

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O 23

55 60 65 70 75 80 85 900

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O

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O

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Na

OK

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22

55 60 65 70 75 80 85 900

0 1.

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P O 2

5

SiO2( )wt. % SiO2

( )wt. %

(wt.

%)

(wt.

%)

(wt.

%)

(wt.

%)

(wt.

%)

(wt.

%)

Syenogranites

Granodiorites

Tuoge Complex

Fig. 6 Harker diagrams for granitoids of the Daxigou and Tuoge complexes.

Tab. 3 Zircon Lu–Hf isotopic compositions for the syenogranite from Daxigou granitoids

Spot Age (Ma) 176Hf/177Hf 1σ 176Lu/177Hf 1σ 176Yb/177Hf 1σ εHf(0) fLu/Hf εHf(t) TDM1 TDM2

GO-01 1849 0.281492 0.000014 0.000886 0.000011 0.030306 0.000169 –45.25 –0.97 –5.14 2.45 2.69GO-02 1834 0.281442 0.000011 0.000418 0.000002 0.013939 0.000137 –47.04 –0.99 –6.69 2.49 2.76GO-03 1829 0.281481 0.000012 0.001018 0.000001 0.032573 0.000105 –45.65 –0.97 –6.14 2.48 2.73GO-04 1821 0.281455 0.000013 0.000475 0.000002 0.015776 0.000033 –46.58 –0.99 –6.58 2.48 2.75GO-05 1834 0.281488 0.000012 0.001045 0.000001 0.034317 0.000117 –45.40 –0.97 –5.82 2.47 2.72GO-06 1830 0.281471 0.000013 0.000655 0.000001 0.022488 0.000080 –46.02 –0.98 –6.04 2.47 2.73GO-07 1799 0.281465 0.000011 0.000366 0.000004 0.012501 0.000192 –46.21 –0.99 –6.57 2.46 2.73GO-08 1813 0.281475 0.000010 0.000917 0.000004 0.031709 0.000227 –45.88 –0.97 –6.61 2.48 2.74GO-09 1824 0.281474 0.000011 0.000571 0.000001 0.019712 0.000086 –45.92 –0.98 –5.97 2.46 2.72GO-10 1869 0.281478 0.000014 0.000751 0.000003 0.027590 0.000050 –45.76 –0.98 –5.03 2.46 2.70GO-11 1827 0.281466 0.000012 0.000581 0.000001 0.020215 0.000065 –46.19 –0.98 –6.18 2.47 2.73GT-01 1774 0.281503 0.000013 0.001312 0.000002 0.041417 0.000174 –44.87 –0.96 –6.92 2.47 2.73GT-02 1792 0.281486 0.000011 0.001004 0.000004 0.033979 0.000182 –45.48 –0.97 –6.77 2.47 2.74GT-03 1820 0.281487 0.000013 0.000882 0.000001 0.030499 0.000060 –45.44 –0.97 –5.96 2.46 2.71GT-04 1796 0.281465 0.000012 0.000601 0.000004 0.020357 0.000114 –46.21 –0.98 –6.93 2.47 2.75GT-05 1776 0.281509 0.000011 0.001669 0.000002 0.055547 0.000179 –44.66 –0.95 –7.10 2.48 2.74GT-06 1828 0.281456 0.000010 0.000591 0.000001 0.020237 0.000052 –46.52 –0.98 –6.51 2.48 2.75GT-07 1835 0.281455 0.000010 0.000739 0.000001 0.025285 0.000078 –46.56 –0.98 –6.58 2.49 2.76GT-08 1770 0.281514 0.000011 0.001752 0.000003 0.059155 0.000161 –44.50 –0.95 –7.16 2.48 2.74GO = group one; GT = group two. εHf(t) = {[(176Hf/177Hf)s − (176Lu/177Hf)s × (eλt − 1)]/[(176Hf/177Hf)CHUR,0 − (176Lu/177Hf)CHUR × (eλt − 1)] − 1} × 10000; s = sample, (176Hf/177Hf)CHUR,0 = 0.282772, (176Lu/177Hf)CHUR = 0.0332, (176Hf/177Hf)DM = 0.28325, (176Lu/177Hf)DM = 0.0384 (according to Blichert-Toft and Albarède 1997; Griffin et al. 2000), t = the crystallization age of zircon, λ = 1.867 × 10–11 a–1 (Söderlund et al. 2004), (176Lu/177Hf)C = 0.015, S and DM are the upper continental crust, the sample and the depleted mantle, respectively.


284

Syenogranites

Granodiorites

Tuoge Complex

Ro

ck

Ch

on

dri

te/

( )b

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

0 1.

1

10

100

1000

Ro

ck

Pri

mit

ive

Ma

ntl

e/

( )a

0.1

1

10

100

1000

Ba Th U K Ta Nb La Ce Pb Pr Sr P NdRb Zr Hf Sm Eu Ti Dy Y Ho Yb Lu

Fig. 7 Primitive mantle-normalized trace-element patterns (a) and chondrite-normalized REE patterns (b) for the Daxigou granitoids and Tuoge Complex. Normalization data are from Sun and McDonough (1989).


285

nearly constant one-stage model ages of 2.45–2.49 Ga and two-stage model ages of 2.69–2.76 Ga. Eight spot analyses for the GT zircons gave εHf(t) values of –7.16 to –5.96, similar one-stage model ages (2.46–2.49 Ga) and two-stage model ages (2.71–2.76 Ga).

Taken together, the studied zircons show a single distribution in εHf(t) values (Fig. 8) with an average of –6.35 (Tab. 3). The Archaean Hf model ages indicate that the studied rocks may have originated from the melting of the Archaean rocks (e.g., TTG) (Fig. 9).

6. Discussion

6.1. Geochemical character, age and likely petrogenesis

As stated above, most samples from the Daxigou granitoids exhibit the mineralogical and geochemical characteristics of I-type granites. Most samples belong to the calc-alkaline series, are fairly rich in SiO2, Na2O (Na2O/K2O > 1 by weight) and have a metaluminous to subaluminous composition. The MgO (0.24–1.66 wt. %) concentrations are obviously lower than those in the aver-age upper crust (2.48 wt. %; Rudnick and Gao 2003), and this precludes their derivation directly from the mantle. Furthermore, the relatively high alkalis suggest the pres-ence of feldspars and/or biotite in the source (Jiang et al. 2005; Zhao XF et al. 2008). The high LREE/HREE ratios, high Sr contents and Sr/Y ratios, low Yb and Y contents and HFSE (Nb, Ta, P and Ti) depletion indicate that the Daxigou granitoids were likely generated at great

depths, with garnet ± apatite, zircon, ilmenite or rutile as the main residual phases. In addition, their low initial εHf(t) values (−7.16 to −5.03, Tab. 3) reveal a continental crustal source. Older Paleoproterozoic rocks (e.g., the Xingditage Complex) can be ruled out as a source on the basis of geochemistry (low SiO2) and isotopic char-acteristics (positive εHf(t): Long et al. 2010). A plausible source would represent Archaean rocks, exposed to the west of the Kuluketage Block, e.g., in the Tuoge Complex (2.65–2.75 Ga, Long et al. 2011a). Indeed, the TDM2 Hf model ages of the Daxigou granitoids and those for the Tuoge Complex are comparable.

In the Nb–Y diagram (Fig. 10a), all of the Daxigou granitoids fall in the field of the volcanic-arc or syn-col-lisional granites. However, in the Rb–(Yb + Nb) diagram (Fig. 10b), almost all of the data plot in the volcanic-arc field. As further evidence, all samples are depleted in Nb, which is typical of granitoids with arc affinity (e.g., Pearce et al. 1984) (Fig. 7b). In all, the combination of field investigations, whole-rock geochemical data, U–Pb ages and zircon Hf isotope data imply that the Daxigou granitoids represent the continental-arc I-type granites, which may have originated by remelting of the TTG (Tuoge Complex) materials.

6.2. Tectonic implications

The age and petrogenesis of the host granitoids have been one of main problems since the discovery of the Daxigou iron–phosphate deposit in the Kuluketage Block. We interpret the newly obtained age of 1830 ± 12 Ma in terms of Paleoproterozoic crystallization of the Daxigou

-14 -11 -8 -5 -20

2

4

6

8

10

12

14

Nu

mb

er

εHf( )T

Fig. 8 Frequency histogram of εHf(t) values for zircons of the syenogranite (SG-2).


286

granitoids. The zoning and chemistry of the dated zircons indicate their magmatic origin.

A series of high-precision ages of Tarim Craton base-ment rocks show that they have mainly experienced two major geological events: about 0.8–1.0 Ga (Zhu et al. 2008; Zhang et al. 2011; Cao et al. 2012) and 2.3–2.8 Ga (Zhang et al. 2012b; Zhang et al. 2013). However, our study indicates that the 1.8–2.0 Ga plutonism may have been important. In addition, the Mesoproterozoic Yangjibulake Group in Kuluketage, which shows effects of greenschist-facies metamorphism, unconformably overlies the Xinditage Group (Zhang et al. 2012a). There-

fore, deducing an important tectonic event at the end of the Paleoproterozoic seems reasonable.

Some 1.9–1.8 Ga ages were recently documented at the margins of the Tarim Craton. However, most of these were ascribed to a metamorphic event (Zhang et al. 2012b). For instance, Wu HL et al. (2012) identi-fied the existence of a 1.85 Ga metamorphic age peak from four metasedimentary rocks in Korla; Zhang et al. (2007b) described a c. 1.9 Ga metamorphic record from the Archaean gneiss and K-feldspar granite in southwest-ern Tarim and Zhang et al. documented c. 1.85–1.80 Ga metamorphic ages from Archaean TTG rocks and the

500 1000 1500 2000 2500 3000-25

-20

-15

-10

-5

0

5

10

15

DXG

KorlaXishankou-1

Xishankou-2

Reworkin

g

Gro

wth

CHUR

DM

Age Ma( )

(a)

chondritic meteoriteslower crust

upper crustZircon

depleted mantle

2.5

Ga

Age (Ma)

(b)

0 1000 2000 3000 4000

0.2835

0.2830

0.2825

0.2820

0.2815

0.2810

0.2805

0.2800

Hf/

Hf

17

61

76

εH

f()

T

Fig. 9 Diagram of εHf(t)–age (a) and (176Hf/177Hf) ratios–age (b) for the syenogranite and related occurrences in the nor-thern Tarim Craton. The pur-ple and gray field in Fig. 9a represent the Neoarchaean and Mesoarchaean accretion--transformation, respectively. 207Pb/235U ages and εHf(t) va-lues for the Korla Gneiss are from Long et al. (2010) and for the Xishankou-1 and Xishan-kou-2 (a Paleoproterozoic gran-itoid in Kuluketage Block) are from (Lei et al. 2012). DXG = Daxigou syenogranite. The data for the development of the main reservoirs are from Long et al. (2010).


287

Paleoproterozoic metamorphic belt around Kuluketage (Zhang et al. 2012a).

Moreover, we have noticed that some tectono-magmatic events occurred there at 1.9–1.8 Ga, related to the assem-bly of Columbia Supercontinent. For instance, Zhang et al. (2007a) identified a 1987 ± 20 Ma inherited component in zircon grains from granodiorite north of Xingdi. Deng et al. (2008) obtained an age of 1916 ± 36 Ma by LA-ICP-MS zircon U–Pb dating of several inherited zircons from a gabbro in the Xingdi Valley of Kuluketage and Cao et al. (2010) an age of 1886 ± 61 Ma by LA-ICP-MS zir-con U–Pb dating of several inherited zircon grains from

Neoproterozoic K-feldspar granite of Dapingliang plutons. Recently, two igneous crystallization ages of 1934 ± 13 and 1944 ± 19 Ma from quartz diorite and granodiorite were obtained west of Kuluketage (Lei et al. 2012). How-ever, the same authors stated that these zircons may have undergone high-temperature metamorphism, considering the nearly identical ages of the cores and metamorphic rims as well as their similar εHf(t) values. We can also see from the above ages that almost all of the zircons are either documented as metamorphic or inherited. In the current study, the 1830 ± 12 Ma age for the Daxigou syenogranite is thus the first reliable crystallization age of the Paleopro-terozoic intrusive rocks in the Kuluketage Block.

Based on the above information, we infer an occurrence of an important Paleoproterozoic (c. 2.0–1.8 Ga) tectono-metamorphic and magmatic event in the Tarim Block. Late Paleoproterozoic collisional orogenic events have been increasingly recognised in Precambrian cratons worldwide and may have ultimately resulted in the formation of the Columbia Supercontinent (e.g., Rogers and Santosh 2002; Zhao GC et al. 2002, 2004; Santosh et al. 2007; Zhao GC et al. 2009; Chen and Xing 2013). Therefore, the Paleo-proterozoic (c. 1.8–1.9 Ga) tectono-magmatic events docu-mented in this study indicate that the Tarim Craton may have taken part in the assembly of the Columbia Super-continent as well. Voluminous I-type granitic plutons have been traditionally considered to form at active continental margins related to oceanic crust subduction (Wilson 1989). Because our zircon dating yielded late Paleoproterozoic crystallization ages, a continental arc-type setting is sug-gested for the northern Tarim at c. 1830 Ma. However, obtaining detailed information about the subduction zone, e.g., its polarity and location of the ocean is currently a challenge because of scarce information on the Kuluketage Block. Much more work is required to reconstruct the plate tectonic history in the Tarim Craton.

7. Conclusions

We can draw the following conclusions from our new field, zircon U–Pb ages and geochemical data: 1. LA–ICP-MS U–Pb zircon dating indicates that the

emplacement and alteration of the Daxigou syeno-granite occurred at 1830 ± 12 Ma and 1798 ± 21 Ma, respectively. This is the first record of a late Paleopro-terozoic to early Mesoproterozoic magmatic event in the Kuluketage area.

2. Based on a combination of field investigations and petrographic, geochronological and geochemical evi-dence, we suggest that Daxigou granitoids belong to Paleoproterozoic continental-arc I-type granites, which may have originated by melting of Neoarchaean TTG (Tuoge Complex) materials.

1 10 100 10001

10

100

1000

Y

Nb

ORG

VAG syn

COLG

+ -

WPG

(a)

1 10 100 10001

10

100

1000

Rb

Yb Nb+

syn COLG-

VAG

ORG

WPG

(b)

Syenogranites

Granodiorites

Fig. 10 Granite discrimination diagrams after Pearce et al. (1984): Nb–Y (a) and Rb–(Yb + Nb) (b). VAG: Volcanic Arc Granite; ORG: Ocean Ridge Granite; WPG: Within-Plate Granite; Syn-COLG: Syn--Collisional Granite; POG: Post-Collisional granite.


288

3. The available data, together with previous studies, de-monstrate that a Paleoproterozoic (c. 2.0–1.8 Ga) tecto-no-magmatic event occurred in the Kuluketage Block. We suggest a continental arc-type tectonic setting in the Kuluketage Block at late Paleoproterozoic times (c. 1830 Ma). The Tarim Craton may have participated in the assembly of the Columbia Supercontinent.

Acknowledgements. This paper benefited from an insightful and helpful review of Editor-in-chief Vojtěch Janoušek. We also acknowledge Milan Kohút for careful editing and valuable comments to the revised manuscript. Critical and constructive comments by two anonymous reviewers were of great help in improving the manuscript. We are grate-ful to Zheng Han, Hu Zhao-chu and Tang Wen-xiu of the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, for their assistance in LA-ICP-MS and Hf isotope determina-tions. All thanks go to Shi Ran for her assistance during the experiments. Geology engineer Xi Guo-qing is ac-knowledged for leading the field work. This research was founded by the 305 Project of State Science and Technol-ogy Support Program (Grant No. 2011BAB06B04-05) as well as the China Postdoctoral Science Foundation projects (Grant No. 2012M521492 and 2013T60758).

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Original paper Petrology and zircon U–Pb dating combined ... · tinental crust are of great importance in understanding the early evolution of the Earth (Condie 1989, 1994; Rudnick

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